Generally, a motor vehicle automatic transmission includes a number of gear elements, such as planetary gear sets, for coupling the transmission's input and output shafts, and a related number of hydraulically actuated torque establishing devices, such as clutches and brakes (the term “torque transmitting device” often used to refer to both clutches and brakes), that are selectively engageable to activate the above mentioned gear elements for establishing desired forward and reverse speed ratios between the input and output shafts. Engine torque and speed are converted by the transmission in response to the tractive-power demand of the motor vehicle.
Shifting from one speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing one or more (off-going) clutches associated with the current or attained speed ratio, and applying one or more (on-coming) clutches associated with the desired or commanded speed ratio. To perform a “downshift”, a shift is made from a low speed ratio to a high speed ratio. The downshift is accomplished by disengaging a clutch associated with the lower speed ratio, and engaging a clutch associated with the higher speed ratio, to thereby reconfigure the gear set to operate at the higher speed ratio. Shifts performed in the above manner are termed clutch-to-clutch shifts and require precise timing in order to achieve high quality shifting.
Some transmission configurations incorporate a hydrodynamic input device, such as a torque converter, positioned between the engine and the transmission. The torque converter is a hydrokinetic fluid coupling employed predominantly to allow the engine to run without stalling when the vehicle wheels and transmission gears come to a stop, and to provide torque multiplication in the lower speed range of the engine. Certain torque converter assemblies include a torque converter clutch, also known as a lockup clutch, operated to provide a bypass mechanism, allowing the engine to circumvent the torque converter and transmit power directly to the transmission.
The various hydraulic subsystems of an automatic transmission, such as the torque transmitting devices, torque converter assembly, torque converter clutch, etc., are typically controlled through operation of a hydraulic circuit, also known as a hydraulic valve system. The hydraulic circuit traditionally engages (actuates) or disengages (deactivates) the various transmission subsystems through the manipulation of hydraulic pressure generated within a transmission oil pump assembly. The valves used in a conventional hydraulic control circuit commonly comprise electro-hydraulic devices (e.g., solenoids), spring-biased accumulators, spring-biased spool valves, and ball check valves.
Ball check valves derive their name from their use of a spherical fluid control element, referred to in the art as a “ball”, to close (seal) and open (unseal) one or more valve ports. Ball check valves are generally used in applications where it is desirable to selectively seal or unseal an opening based upon one or more physical factors such as, for example, pressure gradients, and thereby permit hydraulic fluid to flow in one direction, and prevent fluid flow in another direction. Ball check valves, as discussed herein, should not be confused with ball valves—a distinct type of valve assembly wherein a ball acts as a controllable rotor to stop or direct fluid flow.
In shuttle- or floating-type ball check valves, there are traditionally two independent hydraulic (“inlet”) circuits configured to feed a third (“discharge” or “outlet”) circuit. In this configuration, when the first inlet circuit is “pressurized” and the second inlet circuit is “exhausted”, the check ball is seated against the second inlet port. Accordingly, the second circuit is sealed by the check ball, and the discharge circuit is fed hydraulic fluid by way of the first inlet circuit. Conversely, if the second circuit is thereafter pressurized, and the first circuit exhausted, the check ball will transition or “float” from the seated position against the second inlet port to a seated position against the first inlet port through the generated pressure differential. Consequently, the first inlet circuit is sealed by the check ball, and the discharge circuit is fed hydraulic fluid by way of the second inlet circuit. Additionally, when neither inlet circuit is pressurized, fluid must be allowed to flow from the discharge circuit into at least one of the inlet circuits, which ensures that the discharge circuit is exhausted when neither inlet circuit is pressurized. A typical application in which a shuttle-type ball check valve assembly is used is where the same torque transmitting device (e.g., a clutch) is engaged (i.e., fed hydraulic fluid) from two different ports.